Magnetic Quantum Phase Transitions in Kondo Lattices

The identification of magnetic quantum critical points in heavy fermion metals has provided an ideal setting for experimentally studying quantum criticality. Motivated by these experiments, considerable theoretical efforts have recently been devoted to reexamine the interplay between Kondo screening and magnetic interactions in Kondo lattice systems. A local quantum critical picture has emerged, in which magnetic interactions suppress Kondo screening precisely at the magnetic quantum critical point (QCP). The Fermi surface undergoes a large reconstruction across the QCP and the coherence scale of the Kondo lattice vanishes at the QCP. The dynamical spin susceptibility exhibits $\omega/T$ scaling and non-trivial exponents describe the temperature and frequency dependence of various physical quantities. These properties are to be contrasted with the conventional spin-density-wave (SDW) picture, in which the Kondo screening is not suppressed at the QCP and the Fermi surface evolves smoothly across the phase transition. In this article we discuss recent microscopic studies of Kondo lattices within an extended dynamical mean field theory (EDMFT). We summarize the earlier work based on an analytical $\epsilon$-expansion renormalization group method, and expand on the more recent numerical results. We also discuss the issues that have been raised concerning the magnetic phase diagram. We show that the zero-temperature magnetic transition is second order when double counting of the RKKY interactions is avoided in EDMFT.Comment: 10 pages, 4 figures; references added; as published in JPCM in early September, except for the correction to the legend for Figure

Search for gravitational waves associated with the August 2006 timing glitch of the Vela pulsar

The physical mechanisms responsible for pulsar timing glitches are thought to excite quasinormal mode oscillations in their parent neutron star that couple to gravitational-wave emission. In August 2006, a timing glitch was observed in the radio emission of PSR B0833-45, the Vela pulsar. At the time of the glitch, the two colocated Hanford gravitational-wave detectors of the Laser Interferometer Gravitational wave observatory (LIGO) were operational and taking data as part of the fifth LIGO science run (S5). We present the first direct search for the gravitational-wave emission associated with oscillations of the fundamental quadrupole mode excited by a pulsar timing glitch. No gravitational-wave detection candidate was found. We place Bayesian 90% confidence upper limits of 6.3 x 10^(-21) to 1.4 x 10^(-20) on the peak intrinsic strain amplitude of gravitational-wave ring-down signals, depending on which spherical harmonic mode is excited. The corresponding range of energy upper limits is 5.0 x 10^(-44) to 1.3 x 10^(-45) erg

Local Moment Formation in the Periodic Anderson Model with Superconducting Correlations

We study local moment formation in the presence of superconducting correlations among the f-electrons in the periodic Anderson model. Local moments form if the Coulomb interaction U>U_cr. We find that U_cr is considerably stronger in the presence of superconducting correlations than in the non-superconducting system. Our study is done for various values of the f-level energy and electronic density. The smallest critical U_cr values occur for the case where the number of f- electrons per site is equal to one. In the presence of d-wave superconducting correlations we find that local moment formation presents a quantum phase transition as function of pressure. This quantum phase transition separates a region where local moments and d-wave superconductivity coexist from another region characterized by a superconducting ground state with no local moments. We discuss the possible relevance of these results to experimental studies of the competition between magnetic order and superconductivity in CeCu_2Si_2.Comment: 4 pages. accepted for publication in Phys. Rev.

Proposed method for searches of gravitational waves from PKS 2155-304 and other blazar flares

We propose to search for gravitational waves from PKS 2155-304 as well as other blazars. PKS 2155-304 emitted a long duration energetic flare in July 2006, with total isotropic equivalent energy released in TeV gamma rays of approximately $10^{45}$ ergs. Any possible gravitational wave signals associated with this outburst should be seen by gravitational wave detectors at the same time as the electromagnetic signal. During this flare, the two LIGO interferometers at Hanford and the GEO detector were in operation and collecting data. For this search we will use the data from multiple gravitational wave detectors. The method we use for this purpose is a coherent network analysis algorithm and is called {\tt RIDGE}. To estimate the sensitivity of the search, we perform numerical simulations. The sensitivity to estimated gravitational wave energy at the source is about $2.5 \times 10^{55}$ ergs for a detection probability of 20%. For this search, an end-to-end analysis pipeline has been developed, which takes into account the motion of the source across the sky.Comment: 10 pages, 7 figures. Contribution to 12th Gravitational Wave Data Analysis Workshop. Submitted to Classical and Quantum Gravity. Changes in response to referee comment

Dzyaloshinski-Moriya Interaction in the 2D Spin Gap System SrCu2(BO3)2

The Dzyaloshinski-Moriya interaction partially lifts the magnetic frustration of the spin-1/2 oxide SrCu2(BO3)2. It explains the fine structure of the excited triplet state and its unusual magnetic field dependence, as observed in previous ESR and new neutron inelastic scattering experiments. We claim that it is mainly responsible for the dispersion. We propose also a new mechanism for the observed ESR transitions forbidden by standard selection rules, that relies on an instantaneous Dzyaloshinski-Moriya interaction induced by spin-phonon couplings.Comment: 5 pages, 4 figures, symmetries clarified, added references, (v3) correct addres

First search for gravitational waves from the youngest known neutron star

We present a search for periodic gravitational waves from the neutron star in the supernova remnant Cassiopeia A. The search coherently analyzes data in a 12 day interval taken from the fifth science run of the Laser Interferometer Gravitational-Wave Observatory. It searches gravitational-wave frequencies from 100 to 300 Hz and covers a wide range of first and second frequency derivatives appropriate for the age of the remnant and for different spin-down mechanisms. No gravitational-wave signal was detected. Within the range of search frequencies, we set 95% confidence upper limits of (0.7–1.2) × 10^(−24) on the intrinsic gravitational-wave strain, (0.4–4) × 10^(−4) on the equatorial ellipticity of the neutron star, and 0.005–0.14 on the amplitude of r-mode oscillations of the neutron star. These direct upper limits beat indirect limits derived from energy conservation and enter the range of theoretical predictions involving crystalline exotic matter or runaway r-modes. This paper is also the first gravitational-wave search to present upper limits on the r-mode amplitude

Search for Gravitational-wave Inspiral Signals Associated with Short Gamma-ray Bursts During LIGO's Fifth and Virgo's First Science Run

Progenitor scenarios for short gamma-ray bursts (short GRBs) include coalescenses of two neutron stars or a neutron star and black hole, which would necessarily be accompanied by the emission of strong gravitational waves. We present a search for these known gravitational-wave signatures in temporal and directional coincidence with 22 GRBs that had sufficient gravitational-wave data available in multiple instruments during LIGO's fifth science run, S5, and Virgo's first science run, VSR1. We find no statistically significant gravitational-wave candidates within a [ – 5, + 1) s window around the trigger time of any GRB. Using the Wilcoxon-Mann-Whitney U-test, we find no evidence for an excess of weak gravitational-wave signals in our sample of GRBs. We exclude neutron star-black hole progenitors to a median 90% confidence exclusion distance of 6.7 Mpc